file = open('file.txt', 'r') #в файле 20 строк
for i in range(40):
    line = file.readline()
file.close()

from itertools import cycle, islice

with open('file.txt') as fh:
    for line in islice(cycle(fh), 40):
        ...

with open('file.txt', 'r') as f:
    for i in range(40):
        line = f.readline()

        # Возвращаемся к началу файла
        f.seek(0)

with open('file.txt', 'r') as f:
    for line in f:
        print(repr(line))

    # Возвращаемся к началу файла
    f.seek(0)

    print("Еще раз!")

    for line in f:
        print(repr(line))